The diversity and brilliance of the colors of tropical fish are responsible in part for their popularity in the home aquarium. In the wild, these colors act as signals between fish, signifying anything from sexiness to anger and from fear to health. Colors may change over the course of a fish's life but, to make sure each individual fish within a species is “speaking” the same color “language,” the signal each color pattern conveys remains the same.

Can color reflect a fish's state of health?

Fish color patterns are seldom fixed. Instead, they vary throughout the fish's lifetime and even day to day. When a fish looks “washed out” or drab in comparison to its normal appearance this can often indicate that it is ill or stressed. If steps are taken to address the root cause, the fish will eventually regain its former glory. Bright and well-defined colors show that the fish is in top condition. The aquarist can promote this by paying attention to diet and water chemistry, providing the correct pH and hardness, and keeping nitrates to an absolute minimum with good filtration.

Do fish change their color with their “mood”?

Yes, fish are able to communicate their mood through color. Following a bout of aggression between oscar cichlids, a losing fish will often signal its submission by darkening its color patterns. This may be a signal in its own right or it may simply help the loser blend into the background, making the aggressor less likely to follow up with another attack.

A study on Tilapia revealed 14 distinct color patterns relating to fish mood, each communicating a subtly different message to other fish, ranging from aggression to arousal, and from fright to territoriality.

Why are young fish often a different color to mature ones?

As fish grow and mature, their color patterns may change. Juveniles tend to adopt comparatively drab colors, only to bloom as adults. This is because, in the wild, a small, brightly colored fish is unlikely to survive to adulthood — vivid colors attract predators. However, when the fish reaches maturity, it needs to advertise to potential mates, so it begins to invest in dazzling colors.

How do fish create and change their color?

The colors a fish displays are governed by “chromatophores” — cells that cover the animal and contain color pigments. There are several different types of chromatophores, each responsible for a different element of fish coloration.

Perhaps the most important of these are the melanophores, which contain the black pigment melanin and which affect the darkness of, for instance, a banding pattern. Melanophores, especially, are capable of rapid change in response to the fish's mood or to the environment. If these cells distribute the pigment evenly across themselves, the fish appears dark, but if the pigment is gathered up into dumps, it appears lighter.

Other chromatophores contain other pigments and are responsible for different colors. These include the erythrophores, which hold carotenoids and affect the levels of yellow to red coloration.

Indophores contain guanine and are responsible for both the silvery shimmer of fish such as tinfoil barbs and, because of the way the guanine crystals reflect light, iridescent colors such as the blue stripe of a neon tetra.